Conventionally known fluorine rubber-containing coating compositions suitable for screen printing use a ketone solvent, such as methyl ethyl ketone or methyl isobutyl ketone, for dissolving the fluorine rubber. Such coating compositions are used to form a fluorine rubber coat on one or both sides of a metallic core material having a single layer structure or a laminate structure to produce a metal gasket, such as a cylinder head gasket.
However, the above-mentioned ketone solvents, though excellent in dissolving power for fluorine rubbers, volatilize easily due to high vapor pressure at room temperature. It follows that the coating composition dries during coating operation by screen printing to clog the screen, making it difficult to carry on coating continuously.
If a high-boiling solvent having low volatility is used instead, a fluorine rubber is less soluble, and the resulting coating composition would have an increased viscosity. Besides having poor coating properties, a viscous coating composition is ready to cause clogging of a screen through slight volatilization. If the amount of the solvent is increased in order to prevent clogging, it is difficult for the coating composition having a reduced concentration to provide a thick coat.
Further, where the concentration of a coating composition is increased for formation of a thick coat, the coating composition tends to entrain air bubbles while it is pressed out through the screen perforations and the air bubbles tend to be entrapped in the coating layer. Entrapped air bubbles in a coating layer of a gasket cause not only insufficient sealing performance of the gasket but poor appearance.
In order to overcome these problems, various improvements have been added to the screen mesh size or coating means, such as a doctor and a squeegee, but no satisfactory results have been reached yet.